Bring the milk, 1/4 cup of the sugar, butter, salt and vanilla bean to a gentle boil in a medium saucepan. Remove from heat.
Whisk together the cornstarch and the remaining 1/4 cup of sugar. Add the egg and yolks to the cornstarch and mix into a smooth paste.
Slowly, and in small amounts, whisk a little of the hot milk into a the egg mixture. This is called tempering the eggs, which you need to do to get them to the same temperature of the hot milk in the pan, so they won’t curdle. Once the egg mixture is warm to the touch, pour it back into the milk in the pan.
Return the custard to the stove and bring to a boil, whisking continuously for 2 to 3 minutes. The pastry cream will thicken almost immediately but it is important to cook out the starch so that it isn’t grainy and so your pastry cream won’t separate. Separating pastry cream is when the liquid releases from the cream, easily prevented by cooking for 2-3 minutes! When the pastry cream is done it will be smooth and glossy.
Strain the pastry cream into a shallow container. Because the pastry cream is so thick you will need to press it through the strainer with a rubber spatula.
Cover with plastic wrap pressed directly on the surface of the cream to prevent a skin from forming. Set the container in the freezer for 15 minutes (this cools down the eggs quickly) and then refrigerate for up to a few days.